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Glutaric aciduria type II (riboflavin-responsive)
This rare condition results from issues with enzymes essential for energy production. It can lead to a build-up of harmful substances, potentially causing a range of symptoms, particularly affecting energy levels and muscle function. It typically presents in infancy or early childhood.
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Overview
Glutaric aciduria type II (riboflavin-responsive), sometimes called multiple acyl-CoA dehydrogenase deficiency (MADD), is a rare inherited metabolic disorder. This condition impairs the body's ability to effectively break down specific fats and proteins, which are vital for generating energy. When the body cannot process these substances efficiently, potentially harmful compounds can build up in the blood and urine. The condition exists across a range of severities; forms that respond to riboflavin treatment are generally considered milder than severe neonatal presentations without such a response.
Symptoms & clinical features
The clinical features of riboflavin-responsive glutaric aciduria type II show considerable individual variation but often include muscle weakness (myopathy), especially noticeable during physical activity, and persistent tiredness. Some individuals might experience episodes of low blood sugar (hypoglycaemia), particularly when unwell or after prolonged periods without food. Other potential symptoms can involve brain function (encephalopathy) due to the accumulation of toxic waste products, and heart problems such as cardiomyopathy. Symptoms may worsen during periods of metabolic stress, such as infections or vigorous exercise. Unlike more severe presentations of glutaric aciduria type II, the riboflavin-responsive type typically does not involve significant birth defects.
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Affected organs
This condition primarily impacts organ systems that rely heavily on robust energy production, such as the muscles, liver, and heart. Muscles can become weak because fat metabolism, a key energy source for muscle function, is impaired. The liver may also be affected, leading to difficulties with blood sugar regulation and other metabolic processes. The heart, as an organ with high energy demands, can also be impacted, potentially leading to heart muscle disease (cardiomyopathy). The brain can also be vulnerable to the effects of accumulating metabolic by-products, especially during acute metabolic disturbances.
Risks & severity
The clinical course of glutaric aciduria type II (riboflavin-responsive) varies in severity; it is generally categorised as a less severe form compared to the neonatal onset type. Symptoms can appear during infancy, childhood, or occasionally in adulthood. For the riboflavin-responsive type, the long-term outlook tends to be more favourable, particularly with appropriate medical intervention. Without timely diagnosis and management, there is a risk of metabolic crises, which could lead to serious health complications like severe hypoglycaemia, metabolic acidosis, and neurological damage. However, symptoms can often be managed through dietary changes and riboflavin supplementation. The exact occurrence of this rare condition is not precisely known.
Genetic causes
Glutaric aciduria type II (riboflavin-responsive) is caused by genetic changes in the *ETFDH* gene. The *ETFDH* gene provides instructions for creating the electron transfer flavoprotein dehydrogenase enzyme. This enzyme, found within the mitochondria (often referred to as the 'powerhouses' of cells), plays a vital role in metabolising fats and proteins to produce energy. Specifically, it functions by transferring electrons from other enzymes to the electron transport chain, which is essential for energy generation. When there are pathogenic variants in *ETFDH*, the enzyme's proper function is disrupted, leading to an accumulation of fatty acids and protein metabolites that the body cannot process effectively. Certain variants in *ETFDH* have been observed to respond well to supplementation with riboflavin (vitamin B2), as riboflavin is a crucial component of the enzyme's co-factor, flavin adenine dinucleotide (FAD).
- ETFDH electron transfer flavoprotein dehydrogenaseThe ETFDH gene provides instructions for an enzyme called electron transfer flavoprotein dehydrogenase, essential for breaking down fats and proteins to produce energy within cells.
Inheritance pattern
Glutaric aciduria type II (riboflavin-responsive) is inherited in an autosomal recessive manner. This means that for an individual to develop the condition, they must inherit two altered copies of the *ETFDH* gene - one from each biological parent. Individuals who inherit only one altered copy of the gene are known as carriers. Carriers typically do not experience symptoms themselves but can pass the altered gene on to their offspring. If two carriers have children, there is a possibility with each pregnancy that the child could inherit two altered copies and develop the condition, or inherit one altered copy and become a carrier, or inherit two typical copies and not be affected or a carrier.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
Diagnosis & testing
Diagnosing glutaric aciduria type II (riboflavin-responsive) often begins with recognising clinical symptoms and identifying abnormal results from metabolic screening tests. Initial investigations may include analysis of organic acids in urine and an acylcarnitine profile from blood, which can reveal characteristic metabolic patterns. Genetic testing, specifically looking for pathogenic variants in the *ETFDH* gene, is used to confirm the diagnosis. Genomic testing for suspected inherited metabolic disorders is accessible through the NHS Genomic Medicine Service (GMS) after a referral to clinical genetics. The relevant NHS National Genomic Test Directory R-code for this condition would typically fall under R130, 'Inherited metabolic disorders - gene panel'.
Management & lifestyle
Managing glutaric aciduria type II (riboflavin-responsive) focuses on minimising the build-up of harmful metabolites and ensuring adequate energy supply. This typically involves a specially tailored diet to limit specific fats and proteins while maintaining sufficient calorie intake. High-dose riboflavin (vitamin B2) supplementation is a key part of treatment for the riboflavin-responsive forms, as it can significantly improve enzyme function. During periods of illness or fasting, individuals might require specific interventions, such as intravenous glucose, to prevent metabolic crises. Management is usually overseen by a metabolic specialist and a dietitian, alongside regular monitoring. Genetic counselling also forms an important part of care, providing affected individuals and their families with information and support regarding inheritance patterns and family planning.
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